Timing and data recovery is a key function in a receiver for an optical communication system. The receiver has major functions including receiving the optical data signal and converting it to electrical signal, recovering the timing signal from the received signal, and recovering the noisy input data signal using the recovered clock. Typically the optical data signal in the high speed system is in a non-return to zero (NRZ) format so that it contains no clock information. Using an NRZ format reduces the total bandwidth required by the system. In order to recover the clock signal from an NRZ format signal, a nonlinear circuit is used to detect the timing information. The output of the nonlinear circuit is then fed to a very high Q bandpass filter in order to extract the clock signal and reduce the NRZ data pattern effect. "Q" is a measure of selectivity in filters; the higher the Q, the more selective the filter response becomes. The clock is amplified and limited to suppress the AM component of the signal that is due to the data pattern itself.
As the signal speed exceeds 10Gb/s, the timing recovery becomes difficult and costly, mainly due to the high Q filter and associated components that are required. Data recovery is done by a high sensitivity D type flip flop, which becomes more costly as the signal speed increases. In general, high speed components are costly.